Integrated catalytic process for biomass conversion and upgrading to C 12 furoin and alkane fuel

Article abstract of DOI:10.1021/cs500058p

Report herein is an integrated catalytic process for conversion and upgrading of biomass feedstocks into 5,5′-dihydroxymethyl furoin (DHMF), through self-coupling of 5-hydroxymethyl furfural (HMF) via organocatalysis, and subsequently into n-C₁₂H₂₆ alkane fuel via metal-acid tandem catalysis. The first step of the process involves semicontinuous organocatalytic conversion of biomass (fructose, in particular) to the high-purity HMF. N-Heterocyclic carbenes (NHCs) are found to catalyze glucose-to-fructose isomerization, and the relatively inexpensive thiazolium chloride [TM]Cl, a Vitamin B1 analog, catalyzes fructose dehydration to HMF of good purity (>99% by HPLC), achieving a constant HMF yield of 72% over 10 semicontinuous extraction batch runs. Crystallization of the crude HMF from toluene yields the spectroscopically and analytically pure HMF as needle crystals. The second step of the process is the NHC-catalyzed coupling of C ₆ HMF produced by the semicontinuous process to C₁₂ DHMF; the most effective organic NHC catalyst produces DHMF in 93% or 91% isolated yield with an NHC loading of 0.70 mol % or 0.10 mol % at 60°C for 3 h under solvent-free conditions. The third step of the process converts C₁₂ DHMF to linear alkanes via hydrodeoxygenation. With a bifunctional catalyst system consisting of Pd/C + acetic acid + La(OTf)₃ at 250°C and 300 psi H₂ for 16 h, DHMF has been transformed to liquid hydrocarbon fuel (78% alkanes), with a 64% selectivity to n-C₁₂H₂₆ and an overall C/H/O % ratio of 84/11/5.0.

Full text of DOI:10.1021/cs500058p

Research Article
pubs.acs.org/acscatalysis
Integrated Catalytic Process for Biomass Conversion and Upgrading
to C12 Furoin and Alkane Fuel
Dajiang (D. J.) Liu†,‡ and Eugene Y.-X. Chen*,†
†Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States
‡Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523-1301, United States
S
* Supporting Information
ABSTRACT: Report herein is an integrated catalytic process for
conversion and upgrading of biomass feedstocks into 5,5′-dihydrox-
ymethyl furoin (DHMF), through self-coupling of 5-hydroxymethyl
furfural (HMF) via organocatalysis, and subsequently into n-C12H26
alkane fuel via metal−acid tandem catalysis. The first step of the process
involves semicontinuous organocatalytic conversion of biomass
(fructose, in particular) to the high-purity HMF. N-Heterocyclic
carbenes (NHCs) are found to catalyze glucose-to-fructose isomer-
ization, and the relatively inexpensive thiazolium chloride [TM]Cl, a
Vitamin B1 analog, catalyzes fructose dehydration to HMF of good
purity (>99% by HPLC), achieving a constant HMF yield of 72% over 10 semicontinuous extraction batch runs. Crystallization
of the crude HMF from toluene yields the spectroscopically and analytically pure HMF as needle crystals. The second step of the
process is the NHC-catalyzed coupling of C6 HMF produced by the semicontinuous process to C12 DHMF; the most effective
organic NHC catalyst produces DHMF in 93% or 91% isolated yield with an NHC loading of 0.70 mol % or 0.10 mol % at 60 °C
for 3 h under solvent-free conditions. The third step of the process converts C12 DHMF to linear alkanes via
hydrodeoxygenation. With a bifunctional catalyst system consisting of Pd/C + acetic acid + La(OTf)3 at 250 °C and 300 psi
H2 for 16 h, DHMF has been transformed to liquid hydrocarbon fuel (78% alkanes), with a 64% selectivity to n-C12H26 and an
overall C/H/O % ratio of 84/11/5.0.
KEYWORDS: biomass, fructose, glucose, cellulose, biomass upgrading, organocatalysis, biofuel
to-HMF conversion process,9 whereas the direct dehydration of
fructose to HMF is facile and highly efficient. High to
INTRODUCTION
■
The depletion of fossil fuels has directed society’s increasing
quantitative yields and selectivity of HMF from fructose were
interest toward the use of plant biomass as a sustainable source
achieved in organic solvents (e.g., DMSO),10 ionic liquids
of building blocks for chemicals, materials, and biofuels.1 The
(ILs),9,11 and even in water,12 with or without catalysts.
furan-based compounds, mostly derived from dehydration of
C5 or C6 (poly)sugars, have emerged as the promising platform
chemicals in selective transformations for the following: (1)
In view of the facile fructose-to-HMF route, the development
of an effective glucose-to-fructose isomerization process has
been of great interest. In this context, heterogeneous acidic
benzene derivatives by the Diels−Alder reaction with
dienophiles;2 (2) levulinic acid, γ-valerolactone, and their
zeolite catalysts have been extensively studied for isomerization
of glucose to fructose.13,14 Specifically, Davis et al. showed that
derivatives;3 (3) higher-energy-density fuel intermediates
Sn or Ti modified large-pore zeolite (beta zeolite) was an
effective Lewis acid catalyst for glucose-to-fructose isomer-
ization in water: a product containing 46% glucose, 29%
bearing higher carbon numbers (chain extension) derived
from coupling with other chemicals.4 In particular, 5-hydroxy-
methylfurfural (HMF), a selective hydrolysis product from C6
sugars, has been identified as a versatile intermediate for top-
value-added chemicals, thanks to various functional groups
possibly derived from the 2,5-positioned hydroxyl and aldehyde
fructose, and 8% mannose was obtained after reacting a 45 wt
% glucose solution catalyzed by Sn-Beta for 60 min at 110 °C.13
Enhanced fructose yield from glucose was achieved by Riisager
groups in HMF.5 Glucose, the most abundant C6 sugar, has
et al. using large-pore zeolite Y, especially H-USY (Si/Al = 6),
through the glucose−methyl fructoside−fructose strategy: a
been extensively studied for its direct conversion to HMF in
ILs6 and biphase systems7 catalyzed by Lewis and Brønsted
product containing 28% glucose, 55% fructose, and 4% methyl
fructoside was obtained at 120 °C by a stepwise reaction.14 This
acids, with typically moderate to high yields of 40−80%. Most
novel reaction pathway involves glucose isomerization to
recently, bifunctional catalysts consisting of superhydrophobic
acid and superhydrophilic base were applied for the glucose-to-
HMF conversion, achieving the HMF yield up to 95% in a
THF−DMSO mixed solvent at 100 °C for 10 h.8 Fructose has
been proposed as the key intermediate involved in the glucose-
Received: January 15, 2014
Revised: February 26, 2014
Published: March 18, 2014
© 2014 American Chemical Society
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Figure 1. DHMF as a proposed new C12 “platform chemical” for renewable chemicals, materials, and biofuels.
fructose and subsequent etherification to methyl fructoside in
methanol, followed by hydrolysis to reform fructose after water
addition.
reported for biomass conversion to HMF, the dehydration of
fructose and the isomerization of glucose have remained to be
the two bottlenecks in the production of HMF, and the
separation and purification methods of HMF to produce the
high-purity HMF suitable for subsequent chemical trans-
formations were barely studied.16 Furthermore, although the
HDO process catalyzed by Pt/C and TaOPO4 converted
DHMF to a mixture of C10 (27.0%), C11 (22.9%), and C12
(45.6%) linear alkanes,15b it is preferable to convert the C12
DHMF selectively to C12 alkanes from a viewpoint of atom
efficiency. Accordingly, this work was directed at accomplishing
the following three goals: (a) to investigate possible glucose
isomerization to fructose by organocatalysis; (b) to establish an
efficient, economical, integrated catalytic process for converting
inexpensive biomass feedstocks such as fructose (currently at
∼$32 per 100 g) to DHMF through generation of the high-
purity HMF intermediate; and (c) to identify a bifunctional
HDO catalyst system that can convert C12 DHMF to n-C12H26
alkane more selectively for achieving higher atom efficiency.
In addition to the widely studied metal-based catalyst
systems, the application of organocatalysis in biomass
conversion and upgrading has also come to light recently,
showing the potential of using relatively non- or less toxic,
more environmentally benign, atom-economical, and more
sustainable catalysis for biorefining.15 For instance, we recently
demonstrated that furaldehydes (e.g., furfural, 5-methylfurfural,
and HMF) undergo benzoin-condensation-type self-coupling in
the presence of an N-heterocyclic carbene (NHC) catalyst
under mild conditions.15b Using a room-temperature (RT)
ionic liquid, 1-ethyl-3-methylimidazolium acetate ([EMIM]-
OAc), where 1-ethyl-3-methylimidazol-2- ylidene ([EMIM]
NHC) exists in an equilibrium with [EMIM]OAc and is
stabilized by HOAc, HMF was readily dimerized to C12 5,5′-
dihydroxymethyl furoin (DHMF) in high conversion and good
isolated yield.15c An understanding of the mechanism and
catalytically active species for the [EMIM]OAc-promoted HMF
self-coupling led to a more effective HMF upgrading process
that uses a discrete, stable NHC, 1,3,4-triphenyl-4,5-dihydro-
1H-1,2,4-triazol-5-ylidene (TPT), the process of which afforded
DHMF in an isolated yield up to 95% with 1 mol % of TPT in a
solvent-free process at 60 °C for 1 h.15b
Compared with HMF, DHMF is a higher-energy-density and
perhaps more versatile, C12 “platform chemical”, featuring 12
carbons, 3 hydroxyl groups, 2 substituted furan rings, and 1
carbonyl group (Figure 1). As a C12 fuel intermediate, DHMF
has been converted to oxygenated diesels by hydrogenation,
etherification, and esterification or high-quality alkane fuels by
hydrodeoxygenation (HDO) in water.15b As a furan derivative,
DHMF could be potentially hydrolyzed under acidic
conditions, rending C12 ketones after furan ring opening. In
terms of producing polymeric materials, as a triol, DHMF could
be (co)polymerized into various types of polymers, such as
polyethers, polyesters, polycarbonates, and polyurethanes.
Despite its great potential as a promising C12 platform
chemical, the current process to DHMF relies on the expensive
HMF, which is commercially available (currently sold at ∼$37
per g) but requires further crystallization to give the sufficiently
pure HMF (with a typical 70−80% recovery yield) suitable for
the subsequent organocatalytic upgrading (self-coupling)
process. Although a large number of catalytic processes were
EXPERIMENTAL SECTION
■
Materials, Reagents, and Methods. All syntheses and
manipulations of air- and moisture-sensitive materials were carried
out in flamed Schlenk-type glassware on a dual-manifold Schlenk line
or in an inert gas (Ar or N2) filled glovebox. HPLC-grade organic
solvents were sparged extensively with nitrogen during filling of the
solvent reservoir and then dried by passage through activated alumina
(for Et2O, THF, and CH2Cl2) followed by passage through Q-5-
supported copper catalyst (for toluene and hexanes) stainless steel
columns. Deuterated dimethyl sulfoxide (DMSO-d6) was degassed,
dried over CaH2, filtered, and vacuum-distilled; the dried DMSO-d6
was stored over activated molecular sieves.
NMR spectra were recorded on a Varian Inova 300 (FT 300 MHz)
1
or a Varian Inova 400 MHz spectrometer. Chemical shifts for H and
13C NMR spectra were referenced to internal solvent resonances and
were reported as parts per million relative to tetramethylsilane. HMF
(5-hydroxymethylfurfural) and DHMF (5,5′-dihydroxymethylfuroin)
were analyzed by an Agilent 1260 Infinity HPLC system equipped
with an Agilent Eclipse Plus C18 Column (100 × 4.6 mm; 80/20
water/methanol, 0.6 mL/min, 30 °C) and a UV detector (284 nm).
Sugar contents of the products were measured by an Agilent 1260
Infinity HPLC system equipped with a Biorad Aminex HPX-87H
Column (300 × 7.8 mm; water, 0.6 mL/min, 45 °C) and a RI
detector; under such conditions, possible sugars (e.g., glucose and
fructose) in the reaction mixture can be well separated and quantified.
Hydrodeoxygenation (HDO) reactions were carried out in a Parr 4842
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Figure 2. Structures of chloride salts and NHCs employed in this study.
a
Table 1. Glucose-to-Fructose Isomerization Catalyzed by NHCs
entry
solvent
catalysts
CrCl2
temp (°C)
glucose conversion (%)
fructose yield (%)
fructose selectivity (%)
HMF yield (%)
1
2
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
DMF
100
100
100
100
100
100
80
>99
97
46
63
83
20
83
59
57
26
12
25
58
40
5
CrCl2 + 1IMes
CrCl2 + 2IMes
CrCl2 + 3IMes
IMes
3
15
20
16
33
32
19
4
3
5
6
none
7
IMes
13
24
26
16
1
16
40
45
62
10
83
8
IMes
80
9
DMSO
IMes
80
10
11
12
DMSO
IMes
25
DMSO
TPT
60
DMSO
I-t-Bu
25
20
a
Catalyst loading: 10 mol %. Reaction time: 3 h.
pressure reactor (Parr Instrument Co.). The products were analyzed
either by an Agilent 6890N GC-FID system with a Durabond DB-5
column (60 m, 0.25 mm i.d., 0.25 μm film) or by an Agilent 6890 GC-
MS system equipped with a Phenomenex Zebron ZB-5 ms column (30
m, 0.25 mm i.d., 0.25 μm film). Any DHMF remained in the acetic
acid after HDO was analyzed by HPLC. Elemental analyses were
carried out by Robertson Microlit Laboratories (Ledgewood, NJ).
Sugars (fructose, glucose, and cellulose, Sigma Aldrich), furfural
(Alfa Aesar), HMF (Acros Organics), 5-methyl-2-furaldehyde (Alfa
Aesar), 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride
([TM]Cl, Alfa Aesar), thiamine·HCl (Alfa Aesar), potassium bis-
(trimethylsilyl)amine (KHMDS, Alfa Aesar), tetraethylammonium
chloride (TEAC, Alfa Aesar), KOtBu (Acros Organics), Amberlyst-15
cation exchange resin (Acros Organics), and Dowex M43 Anion
Exchange Resin (Supelco) were used as received. The ionic liquid 1-
ethyl-3-methylimidazolium chloride ([EMIM]Cl, Sigma Aldrich) was
dried under vacuum at 100 °C for 24 h, followed by repeated
recrystallization from CH2Cl2 and hexanes at room temperature.
Catalysts for HDO reactions, including Pd/C (10 wt % Pd on
activated carbon, wet support, Sigma Aldrich), La(OTf)3 (Alfa Aesar),
were used as received. NHCs (N-heterocyclic carbenes), 1,3-ditert-
butylimidazol-2-ylidene (ItBu, TCI America), and 1,3-bis(2,4,6-
trimethylphenyl)-1,3-dihydro-2H-imidazole-2-ylidene (IMes, Sigma
Aldrich) were used as received. Literature procedures were used to
prepare 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene
(TPT).17 [TM]-NHC and thiamine-NHC were preformed through
deprotonation of [TM]Cl and thiamine·HCl by KHMDS in THF,
followed by extensively washing with hexanes to remove an excess
amount of KHMDS. Polymeric NHC precursor P[BVIM]-CO2 and
polymeric NHC P[iPrVIM] (Figure 2) were synthesized from their
respective bromide salts following literature procedures.18
DMSO was added. After it was stirred at room temperature for 3 h, the
solution was quenched and diluted by 25 mL of water for an HPLC
test. For reactions carried in ILs, glucose and fructose contents were
measured by HPLC analysis (RI detector) after the diluted ILs phase
passing through the cation and anion exchange columns to discharge
ILs.
Integrated Three-Step Process of Fructose Conversion to
C12 Alkane Fuel. Experimental procedures were briefly described
herein, and detailed descriptions for each step were included in the
Supporting Information. In the first step, the commercial fructose was
converted to the high-purity HMF by a semicontinuous process
followed by crystallization. For the semicontinuous extraction of HMF
by the THF/[TM]Cl biphasic system, 2.0 g of fructose was mixed with
10 g of [TM]Cl in a 75 mL pressure glass reactor, to which 20 mL of
THF was added. The reactor was capped and heated at 120 °C for 1.5
h under stirring. When the glass reactor was cooled to room
temperature after the reaction, the THF phase was collected. For each
of the further 9 batches, 2.0 g of fructose and 20 mL of THF were
loaded to the reactor, which was capped and heated at 120 °C for 1 h.
The THF phases obtained were combined and decolored by activate
carbon. Analysis of the THF phases by HPLC showed HMF was
formed in good purity (>99% by HPLC), perhaps sutiable for some
applications without further purification, but in this work, HMF was
subsequently isolated as crystals according to the following procedures.
THF was removed by evaporation under vacuum, and the remaining
liquid product was redissolved in toluene and crystallized to afford
HMF needle crystals. A typical yield for the isolated high-purity,
crystalline HMF was about 30−36%, depending on the reaction scale.
In the second step, the purified HMF was upgraded into DHMF
catalyzed by TPT or in situ-generated [TM] NHC. The HMF
umpolung catalyzed by TPT was performed similarly as previously
reported.15 HMF (3.0 g, 24 mmol) was premixed with [TM]Cl (0.60
g, 10 mol % to HMF) and KOtBu (0.40 g, 15 mol % to HMF) in a 20
mL vial. The sealed vial was placed in a temperature-controlled orbit
Glucose Isomerization to Fructose Catalyzed by NHCs. In a
typical reaction, 20 mg of glucose was loaded into a 5 mL vial, to
which 3.3 mg of IMes (10 mol % relative to glucose) in 0.5 mL dry
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shaker (80 °C, 300 rpm) and heated for 3 h. The crude product was
purified by silica-gel chromatography. After removing the solvent
under vacuum, DHMF (1.6 g) was isolated in 53% yield. However, the
same reaction catalyzed by TPT (0.7% mol) achieved a high yield of
93%.
Scheme 1. Proposed Proton Transfer Mechanism for
Isomerization of Glucose to Fructose Catalyzed by NHCs
In the third step, DHMF was converted to n-C12H26 alkane fuel by
HDO. DHMF (0.25 g, 0.99 mmol) was dissolved in 40 mL of glacial
acetic acid and transferred to a Parr pressure reactor. To this reactor
was added water-wetted Pd/C (0.12 g, ∼12 mol % Pd relative to
DHMF) and La(OTf)3 (0.25 g, 43 mol % relative to DHMF). The
reactor was purged with H2 for 15 min and heated at 250 °C for 16 h
under 300 psi H2. After completion of the reaction, an aliquot was
analyzed by GC-MS, GC-FID, and HPLC. To separate the HDO
products, the acetic acid solution was concentrated and extracted by
hexanes; the extracts were dried by anhydrous MgSO4, and the solvent
was removed under vacuum. The resulting oil product was subjected
to elemental analysis.
observed upon the initial mixing of I-t-Bu with glucose, through
the open-chain form (A) of glucose. Deprotonation of the C-2
proton of A by an NHC base affords the enolate intermediate
(B). Through further proton transfer steps involving enolate
intermediates, fructose is formed and the NHC catalyst is
regenerated (Scheme 1). Hence, in this NHC-catalyzed
glucose-to-fructose isomerization, the yield and selectivity of
fructose are strongly related to the basicity of NHCs. As
expected, besides TPT, other weaker bases (e.g., NaOH and
DBU) achieved only less than 5% of fructose yield under
similar conditions. Noteworthy is that this reaction cycle is also
reversible under the NHC-catalyzed conditions: when starting
from fructose, a 30% glucose yield and a 61% glucose selectivity
were observed at 60 °C for 1 h catalyzed by IMes.
Recently, Chi et al. reported an interesting finding that
NHCs can catalyze the retro-benzoin condensation of glucose
to form acyl anion intermediates through C−C bond cleavage
of glucose.20 In their case, glucose was used as the acyl anion
resource for subsequent Stetter reaction with chalcone. A high
yield of Stetter product (80%) from the glucose and chalcone
reaction was formed, which was catalyzed by in situ-generated
thiazolium-based NHCs in 30 min at 130 °C under microwave
irradiation. Interestingly, they revealed that the imidazolium-
based NHCs were not effective for the retro-benzoin
condensation of glucose under their reaction conditions, and
the current results showed that the imidaozlium-based NHCs
such as I-t-Bu and IMes are effective for catalyzing the glucose-
to-fructose isomerization even at room temperature.
HMF from Biomass (Fructose, Glucose, and Cellulose). To
explore inexpensive alternatives to commonly used imidazo-
lium-based ILs for biomass conversion to HMF, we found that
[TM]Cl, an analog of thiamine (Vitamin B1), which is about
3.5 times less expensive than the typically employed IL
[EMIM]Cl, can catalyze fructose conversion to HMF in high
efficiency. Although the melting point of [TM]Cl (142−144
°C) is well above 100 °C, a homogeneous solution is readily
formed from the mixture of fructose and [TM]Cl upon heating
to 100 °C, affording HMF in 56% yield after heating the
mixture at this temperature for 1 h (Table 2, entry 2). At higher
temperatures of 120 and 130 °C, the HMF yield increased to
72% (entry 3) and 68% (entry 4), respectively, which was
comparable with that achieved by [EMIM]Cl (entry 1). As for
the thiamine·HCl (B1) catalyzed fructose-to-HMF conversion,
no homogeneous solution was formed by heating the mixture
of thiamine·HCl and fructose, and no HMF was produced.
However, by adding water (50 wt % relative to thiamine·HCl)
to increase fructose solubility in thiamine·HCl, a moderate
HMF yield of 56% was obtained at 120 °C after 1 h (entry 5).
RESULTS AND DISCUSSIONS
■
Step 1: Semi-Continuous Organocatalytic Process for
High-Purity HMF Production. Organocatalyzed Glucose-
to-Fructose Isomerization. In view of facile conversion of
fructose to HMF, at the outset of this project we sought to
develop organocatalyzed isomerization of glucose to fructose,
thus possibly enabling conversion of glucose to HMF via
organocatalysis. This possible route was prompted by our
recent study, which revealed that NHCs (e.g., IMes) actually
poisoned the CrCl2 catalyst for the glucose-to-HMF conversion
in [EMIM]Cl.19 When a stoichiometic amount IMes (1 equiv
to CrCl2) was added, HMF yield decreased from 58% to 40%
(Table 1, entries 1 and 2); further addition of IMes led to
complete catalyst poisoning toward HMF production. Alter-
natively, we observed up to 20% of fructose yield and 32%
fructose selectivity from glucose when a superstoichiometic
amount (2 or 3 equiv to CrCl2) of NHCs was added (entries 3
and 4). Control experiments (entries 1, 5, and 6) clearly
showed the following results: (1) the glucose-to-fructose
isomerization was not achieved in the IL alone (entry 6); (2)
in the presence of CrCl2, the isomerization was achieved in high
efficiency, but with a faster rate for further fructose dehydration
to HMF, the overall reaction afforded HMF in 58% yield but
no fructose (entry 1); and (3) IMes catalyzed the isomerization
in [EMIM]Cl while impeding fructose dehydration to HMF,
thus forming fructose in 16% yield but no HMF (entry 5).
Although there was no further dehydration of fructose to HMF,
the fructose selectivity was only 19%.
In the search for ways to possibly increase the fructose yield
and selectivity, we varied the temperature, solvents, and NHC
catalysts. It was revealed that under the same conditions (10
mol % IMes, 80 °C, 3 h), fructose yield increased from 13% to
24% and 26% by changing the IL [EMIM]Cl to organic
solvents DMF and DMSO, with an increase in fructose
selectivity from 16% (entry 7) to 40% and 45%, respectively
(entries 8 and 9). Interestingly, this isomerization can even be
carried out at RT (entry 10), achieving a similar fructose yield
(16%) but relatively high fructose selectivity (62%). For other
types of NHCs, the weaker base TPT was unable to catalyze
the glucose-to-fructose isomerization in high efficiency (entry
11), although the stronger base I-t-Bu rendered a higher
fructose yield (20%) and selectivity (83%) (entry 12).
Accordingly, this glucose-to-fructose isomerization catalyzed
by NHCs is proposed to follow the base-catalyzed isomer-
ization mechanism (Scheme 1).13b A fast mutarotation from α-
D-glucose to β-D-glucose with a molar ratio of 1:1.8 was
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a
Table 2. Biomass Conversion to HMF by an Organic or Metal Catalyst
entry
biomass
ILs
catalyst
temp (°C)
time (h)
HMF yield (%)
1
2
3
4
fructose
fructose
fructose
fructose
fructose
glucose
glucose
glucose
glucose
cellulose
cellulose
cellulose
[EMIM]Cl
[TM]Cl
[TM]Cl
[TM]Cl
thiamine·HCl
[EMIM]Cl
[TM]Cl
[TM]Cl
[TM]Cl
[TM]Cl
TEAC
100
100
120
130
120
100
120
120
130
140
130
130
1
1
74
56
72
68
56
58
51
45
47
1
1
0.5
1
b
5
6
7
CrCl2
3
CrCl2
1
8
CrCl3·6H2O
CrCl3·6H2O
CrCl3·6H2O
CrCl3·6H2O
CrCl3·6H2O
1
9
0.5
1
10
c
11
0.5
0.5
27
d
12
TEAC
42
a
b
c
d
Biomass/ILs 1/5 wt %, 10 mol % catalyst loading. 50 wt % water relative to thiamine·HCl was added. Cellulose/ILs 1/10 wt %. Cellulose/ILs 1/
10 wt %, 50 wt % water relative to cellulose was added.
For glucose conversion to HMF, an isomerization catalyst
(e.g., CrCl2, CrCl3·6H2O) is required. In the presence of 10
mol % of CrCl2, HMF was obtained in 51% yield from glucose
in [TM]Cl at 120 °C after 1 h (entry 7), which was comparable
with the glucose conversion in [EMIM]Cl at 100 °C after 1 h
(entry 6). A lower HMF yield (45%) was achieved by CrCl3·
6H2O (entry 8), whereas a slightly higher HMF yield (47%)
was obtained within 0.5 h by increasing the reaction
temperature to 130 °C (entry 9). Direct conversion of cellulose
to HMF in [TM]Cl and TEAC was also investigated. Although
a low HMF yield was observed with [TM]Cl and CrCl3·6H2O
even at 140 °C (entry 10), 27% of HMF was achieved with
CrCl3·6H2O in TEAC at 130 °C after 0.5 h (entry 11).
Addition of water (50 wt % relative to cellulose) enhanced
HMF yield to 42% (entry 12), presumably because water
facilitated the initial hydrolysis of cellulose to reducing sugars.
High-Purity HMF from Fructose by Semicontinuous
Extraction. On the basis of the above established one-pot
conversion of fructose to HMF in [TM]Cl, semicontinuous
extraction between the organic and [TM]Cl phases was
adopted to separate HMF from [TM]Cl. THF was selected
as the organic solvent, considering the high solubility of HMF
and low solubility of [TM]Cl in this solvent. Moreover, THF
can continuously remove water generated from fructose
dehydration to HMF, thus suppressing HMF rehydration to
organic acids and humins. During the initial five batches, HMF
yield extracted by THF gradually increased to 70%, after which
it remained nearly constant (Figure 3). It is noteworthy that
this semicontinuous process for HMF production from fructose
is efficient and robust: (1) 72% HMF yield and 74% of HMF
selectivity were achieved over the 10 batch runs, and 90% of
HMF formed was extracted by THF; (2) no apparent loss of
the HMF yield was observed even after 10 batches, with a
consistent HMF yield after the fifth batch; and (3) for each of
subsequent batches, fructose and THF were loaded to the
reactor, and no more [TM]Cl was needed. After completion of
the reaction, the product HMF was isolated, and the collected
THF can be recycled. Overall, this semicontinuous process is
efficient and economical, representing a net transformation of
the inexpensive feedstock fructose to the high-value platform
chemical HMF, which is currently about 100 times more
expensive than the starting fructose.
Figure 3. HMF yields obtained from fructose conversion in [TM]Cl at
120 °C for 1 h (1.5 h for the first batch) by semicontinuous extraction
with THF. Data were shown as the average value of at least two runs
with typical errors within 3%. Lower yields of the initial two batches
were presumably due to absorption of HMF by [TM]Cl until it
reaches a saturation point.
During the initial four batches, HMF yield gradually reached
the maximum at 50%, after which it decreased to 44% at the
sixth batch (Figure 4), presumably due to the Cr catalyst loss
upon repeated extractions with THF. After six batches, HMF
was obtained from glucose in 49% yield and 52% selectivity,
and 92% of HMF formed was extracted by THF. When EtOAc
was used as the organic solvent, HMF yield (31%) was
consistently lower than that extracted by THF, although no loss
of catalyst activity was observed after the fifth batch. For the
cellulose-to-HMF conversion, semicontinuous extraction of
HMF by THF/TEAC gave a low HMF yield. Nevertheless,
extraction with water/EtOAc (1/4 v/v) after the reaction
achieved 39% of HMF yield and 94% of HMF recovery.
The HMF product obtained from fructose dehydration by
the above-described semicontinuous extraction was sufficiently
pure (>99% by HPLC, Figure S2), presumbably sutiable for
most applications without further purification. However, there
were still a small amount of impurities (e.g., [TM]Cl, H2O, and
The above semicontinuous process in [TM]Cl and extraction
with THF was also applied to the glucose-to-HMF conversion
in the presence of CrCl3·6H2O (10 mol % relative to glucose).
1
organic acids) present in the product according to H NMR
analysis (Figure 6a). To produce the spectroscopically and
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1
route 1), but H NMR analysis showed the presence of only a
tiny amount of an impurity appeared as a broad signal at ∼2.8
ppm (Figure 6b). The second route was recrystallization of the
crude HMF product (Figure 5, route 2). Screening of solvents
suitable for recrystallization led to toluene, which yielded the
analytically and spectroscopically pure HMF as needle crystals
(Figure 6c). In this purification route, the crude HMF obtained
from the above semicontinuous process was dissolved in hot
toluene (50−60 °C) and crystallized in a freezer overnight,
affording the pure HMF as needle crystals (yield 30−36%). A
step of decoloration with activated carbon can be added to treat
the crude HMF before crystallization. The crystallized HMF
was used for further umpolung to DHMF catalyzed by either
TPT or other NHC catalysts.
Step 2: HMF Coupling to C12 DHMF by Organo-
catalysis. We have previously shown that furaldehydes such as
furfural, 5-methylfurfural, and HMF can be efficiently self-
coupled in a 100% atom-economical manner into furoin, 5,5′-
dimethylfuroin (5,5′-DMF), and C12 furoin (DHMF),
respectively, through an organocatalytic umpolung process
(i.e., self-condensation) involving an enol intermediate
(Scheme 2).15 In this study, we explored the NHC catalysts
derived from readily available, inexpensive precursors, including
[TM] NHC, thiamine NHC, and poly(NHC)s. The more
expensive, proven efficient TPT catalyst was also used for
comparison and for examining the purity and suitability of the
HMF produced from fructose by the current semicontinuous
process. The results of this investigation were summarized in
Table 3.
Figure 4. HMF yields obtained from glucose conversion in [TM]Cl/
CrCl3·6H2O (10 mol %) at 130 °C for 0.5 h (1 h for the first batch) by
semicontinuous extraction with THF. Data were shown as the average
of at least two runs with errors within 3%.
analytically pure HMF, we investigated two purification routes.
This first route was extraction with diethyl ether and
subsequent solvent removal under vacuum, affording the
purer HMF (54% yield) as dark yellow liquid (Figure 5,
The results showed that TPT is still the most efficient
catalyst. With a low catalyst loading of 0.70 mol % at 60 °C for
1 h, the HMF self-condensation in neat gave DHMF in 93%
isolated yield (Table 3, entry 1). The catalyst loading can be
further lowered to only 0.10 mol %, whereas DHMF can still be
isolated in high yield (91%, entry 2). These results clearly
demonstrated that the HMF produced from fructose by our
semicontinuous process was in high purity and readily suitable
for this critical chain-extension coupling step by organo-
catalysis.
Figure 5. Integrated process for the production of the spectroscopi-
cally and analytically pure HMF from fructose catalyzed by [TM]Cl.
Figure 6. 1H NMR (CDCl3) of HMF (δ 9.61, 7.21, 6.52, 4.72 ppm) by two different purification routes: (a) crude HMF; (b) HMF after extraction
with diethyl ether (cf., Figure 5, route 1); and (c) HMF needle crystals after recrystallization from toluene (cf., Figure 5, route 2).
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the HMF umpolung reaction, under otherwise identical
conditions.
Scheme 2. Solvent-Free NHC-Catalyzed Self-Condensation
of HMF to DHMF and Depicted Umpolung Catalytic Cycle
Step 3: HDO of DHMF to n-C12H26 Alkane Fuel by
Metal−Acid Tandem Catalysis. Previously, we reported a
method of converting DHMF to a mixture of linear alkanes
consisting of C10 (27.0%), C11 (22.9%), and C12 (45.6%) using
a bifunctional catalyst system containing Pt/C and TaOPO4 in
water under 500 psi H2 at 300 °C.15b Recently, a highly
selective HDO process utilizing acetic acid, Pd/C and
La(OTf)3 was developed, which converted cross-aldol con-
densation products (e.g., furan aldehydes and enolizable
ketones) into their corresponding alkanes under relatively
mild conditions (200 °C and 300 psi H2).21 In light of this
important development, we employed this metal/acid
(Brønsted + Lewis acid) catalyst system, Pd/C + acetic acid
+ La(OTf)3, for the HDO of DHMF, the results of which study
were shown in Table 4.
The initial HDO process carried out at 200 °C and 300 psi
H2 gave a selectivity of 26% for n-C12H26 and 1.5% for n-
C11H24, and the HDO products contained 31% alkanes
(including branched and cyclic C11 and C12 alkanes) and 69%
oxygenated compounds (entry 1). Although a low selectivity of
n-C12H26 was obtained from this trial run, the majority of the
oxygenated compounds were found to be C12 esters and
ketones, which could be further converted to C12 alkanes under
modified conditions. Hence, two reaction parameters, H2
pressure and reaction temperature, were varied. By increasing
the H2 pressure from 300 to 500 psi, even lower n-C12H26
selectivity (22%) was obtained (entry 2). On the other hand,
increasing the reaction temperature from 200 to 250 °C while
keeping the H2 pressure the same (300 psi) resulted in a
significant increase of the n-C12H26 selectivity from 26% to 64%
(entry 3), under otherwise identical conditions. The fuel
produced at this temperature contained 78% alkanes and 22%
oxygenated compounds (Figure 7), and the elemental analysis
showed 84% C, 11% H, and 5.0% O. Using 5,5′-DMF as a
comparable substrate to DHMF, the HDO process at 200 °C
and 300 psi H2 for 16 h afforded alkane fuel with a selectivity of
66% for n-C12H26 (43% yield) and 1.5% for n-C11H24 without
the formation of C12 esters (entry 4, Figure S5). Meanwhile, the
In comparison, [TM]-NHC produced DHMF in only
moderate isolated yield of 60%, even with a 10 mol % catalyst
loading (entry 3). Using the in situ-generated [TM]-NHC
through deprotonation of [TM]Cl by KO-t-Bu, a comparable
DHMF yield (53%, entry 4) was obtained after purification by
silica-gel chromatography. However, thiamine-NHC exhibited
no catalytic activity toward DHMF formation under similar
conditions (entry 5). We also employed poly(NHC)s as
catalysts for HMF coupling. Specifically, the masked poly-
(NHC), P[BVIM]-CO2, and the preformed poly(NHC), P[i-
PrVIM], produced DHMF in only 32% and 18% yields (by 1H
NMR), respectively. These results were sharply different from
those obtained by adopting poly(NHC)s to catalyze the
umpolung of benzaldehyde, in which up to 92% yield of
benzoin was achieved at room temperature for 24 h.18b This
difference can be attributed to the acidic proton in the hydroxyl
group of HMF which impedes the catalytic activity of
poly(NHC)s. Supporting this reasoning, furfural behaved
much like benzaldehyde in the umpolung reaction catalyzed
by NHCs. For example, with a low loading of [TM]-NHC (1
mol %), furfural was readily coupled to furoin in 86% isolated
yield, as compared to a much lower yield of 60% in the case of
Table 3. DHMF Yields from the HMF Umpolung Reaction Catalyzed by NHCs
entry
solvent
NHCs
NHC loading (mol %)
temp (°C)
time (h)
DHMF (%)
1
2
3
4
5
6
7
none
TPT
TPT
0.7
0.1
10
10
10
10
10
60
60
80
80
60
80
80
1
3
3
3
3
3
3
93
91
60
53
0
none
toluene
none
[TM]-NHC
in situ [TM]-NHC
thiamine-NHC
P[BVIM]-CO2
P[i-PrVIM]
toluene
THF
32
18
THF
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a
Table 4. Analytical Results of Liquid Fuels after HDO in Acetic Acid
isolated fuels
entry
furoins
temp (°C)
pressure (psi)
n-C11H24 (%)
n-C12H26 (%)
alkanes (%)
oxygenated (%)
C (%)
H (%)
O (%)
1
2
3
4
DHMF
DHMF
DHMF
5,5′-DMF
200
200
250
200
300
500
300
300
1.5
26
22
64
66
31
25
78
75
69
75
22
25
76
73
84
83
11
10
11
13
13
17
5.7
1.5
5.0
4.0
a
Reaction conditions: reaction time, 16 h; solvent, acetic acid, 40 mL; catalysts, Pd/C + La(OTf)3. In all cases, furoins were completely converted.
Branched and cyclic C11 and C12 were included in the calculation of alkane selectivity. 5,5′-DMF = 5,5′-dimethylfuroin.
[TM]Cl and the extraction solvent THF can be readily recycled
for subsequent batch runs.
The second step of the process is the organocatalytic
coupling of the C6 HMF produced by the above semi-
continuous process to C12 DHMF in the presence of an NHC
catalyst. Among several types of NHCs investigated for this
coupling reaction, TPT was proven to be most effective,
achieving 93% or 91% isolated yield of DHMF in the presence
of 0.70 mol % or 0.10 mol % TPT at 60 °C for 3 h (in a
solvent-free process). The [TM]Cl-derived NHC is less
effective, affording DHMF in 60% isolated yield with a 10
mol % catalyst loading. The two poly(NHC)s investigated are
least effective, producing DHMF in much lower yields of only
18−32%. Overall, the TPT-catalyzed HMF self-condensation
(chain-extension) process to DHMF can be regarded as a
greener process, with 100% atom-economy, solvent-free
operation, and near quantitative yield from an organocatalytic
reaction.
The third step of the process is the hydrodeoxygenation of
C12 DHMF to linear alkanes as a potential high-quality
hydrocarbon fuel. With the metal (Pd/C)/acid (La(OTf)3)
catalyst system, the HDO of DHMF in acetic acid at 250 °C
and 300 psi H2 for 16 h afforded liquid hydrocarbon fuel (78%
alkanes) with a 64% selectivity to n-C12H26 and an overall C/
H/O % ratio of 84/11/5.0.
Figure 7. GC-MS chromatogram of the crude products produced by
the HDO of DHMF in acetic acid catalyzed by Pd/C+ La(OTf)3 at
250 °C under 300 psi H2 for 16 h (Table 4, run 3).
isolated fuel exhibited similarly high C and H molar ratios (83%
and 13%) and a correspondingly low O ratio of 4.0%.
CONCLUSION
■
In summary, we have developed an integrated catalytic process
for biomass conversion and upgrading to C12 DHMF and
subsequently to n-C12H26 alkane by organocatalysis and
subsequent metal−acid tandem catalysis. The first step of the
process involves semicontinuous organocatalytic conversion of
biomass (fructose, in particular) to the high-purity HMF. In
this step, NHCs were found to be effective in catalyzing
glucose-to-fructose isomerization, even at room temperature.
Particularly, treatment of glucose with 10 mol % of I-t-Bu in
DMSO for 3 h afforded fructose in 20% yield with a 83%
selectivity. For biomass conversion to HMF, the inexpensive
[TM]Cl, a Vitamin B1 analog, was utilized for fructose and
glucose dehydration to HMF, achieving a 72% HMF yield from
fructose (no metal catalysts) and up to 51% yield from glucose
(in the presence of additional chromium(II) or (III) catalyst) at
120 °C for 1 h. The semicontinuous extraction process
developed for the HMF production from fructose is efficient
and robust: a constant HMF yield of 72% was achieved over 10
batch runs. The HMF product obtained from the semi-
continuous process is sufficiently pure (>99% by HPLC),
presumably sutiable for most applications without further
purification. On the other hand, further purification by
extraction with diethyl ether gave the purer HMF (54%
yield) as a liquid or by crystallization from toluene afforded the
spectroscopically and analytically pure HMF as needle crystals.
Overall, this semicontinuous process represents a net trans-
formation of the inexpensive feedstock fructose to the higher-
value platform chemical HMF (∼100-fold price increase) as the
ASSOCIATED CONTENT
* Supporting Information
Detailed experimental procedures and additional figures. This
information is available free of charge via the Internet at http://
pubs.acs.org/.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: eugene.chen@colostate.edu. Fax: (+1) 970-491-1801.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy
Office of Basic Energy Sciences, grant DE-FG02-10ER16193.
D.J.L. acknowledges Prof. Susan James as a coadvisor.
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